In recent years, a diversity of steel products has been demanded, which frequently requires the hot rolling of steel products under severe conditions. The hot rolling under severe conditions often causes the seizure between the rolls for hot rolling (i.e. hot rolling rolls constituted essentially of iron-based materials and hereinafter referred to simply as "rolls") and the steel products to be rolled. As the wear of the rolls advances, the rolls suffer deterioration on the surfaces, thus, presenting the problem that the deterioration of the rolls transfers to the surface of the steel products being rolled. For instance, where H beams are hot rolled by means of a universal mill, the seizure occurs between the rolls and the H beams especially at a portion where a sliding velocity relative to the H beams becomes great or at a portion where a normal stress becomes locally high, thereby causing problems. To avoid this, it is inevitable that frequent care must be done for the rolls and the rolled steel products. This takes uncountable time and cost.
Especially, where stainless steel H beams are hot rolled, the seizure is very likely to occur. The countermeasures therefore have been extensively studied from different angles. For instance, high chromium cast iron or high speed steels have been developed from the standpoint of materials for the rolls. However, when these materials are used as the rolls, it is not always possible to completely prevent the seizure. At present, for the prevention of the seizure, lubricants such as graphite compounds have to be used.
To prevent the seizure, an attempt has been made to treat the roll surfaces. For instance, a roll is covered on the surface with Cr by a chromium-plating method, TiN by a CVD method (i.e. a chemical vapor deposition method), and TiC by a PVD method (i.e. a physical vapor deposition method).
However, the time and cost required for these treatments are vast. When using the rolls which have been subjected to the above-mentioned surface coating treatments, the film on the roll is liable to peel off under such severe conditions as of the hot rolling. In this sense, any stable anti-seizing property and wear resistance have not been developed to date.
Extensive studies have also been made on lubricants for hot rolling rolls. Among known lubricants, organic lubricating oils often undergo shortage of oil film or combustion, so that satisfactory lubricating effects cannot be expected. JP-A No. 5-212419 discloses a technique wherein graphite is deposited on roll surfaces as a lubricant by means of a combustion burner using a hydrocarbon fuel. The method proposed in this publication is certainly effective in preventing the seizure. However, the use of graphite has the problem that steel products to be rolled undergo "carburization" under hot (high temperature) conditions because of the use of graphite and that steel products to be rolled are apt to slip on rolling. In addition, the graphite particles scatter to worsen the working environment.
JP-B2 No. 3-25241 discloses a technique wherein, while a mixed liquid paint, composed of an anti-seizing agent, consisting of metal oxide powder and a binder is continuously supplied to and coated onto the peripheral surface of a disk roll-type guide shoe, steel products are pierced. The method proposed in this publication is effective in preventing the seizure. However, because the anti-seizing agent is fixedly deposited on the roll surface, water glass has to be used as the binder. If the above-mentioned mixed liquid paint is not uniformly coated onto the roll surface, surface depression takes place in the surface of the steel products being rolled, thereby causing the surface condition to be degraded. The metal oxide powder used as the anti-seizing agent does not dissolve in the water glass binder. If this powder is allowed to stand under conditions where it is mixed with water glass, it deposits or settles down. This may cause a coating nozzle to be clogged.
Studies have now been made on inorganic solid lubricants such as molybdenum disulfide and glass. Among these lubricants, some lubricants show good lubricity under high temperature conditions. However, like graphite, coating conditions and the removal of residual lubricants are difficult, with the attendant apprehension that an adverse influence is made on the product properties and the working environment. In addition, the lubricants act to lower the coefficient of friction, thus presenting the problem of slipping, which occurs during the course of the hot rolling.
On the other hand, it is known that calcium carbonate has a good effect as a solid lubricant. However, calcium carbonate is difficult to coat. When it is coated in the form of solid powder, dust generates to worsen the working environment. Like inorganic solid lubricants such as molybdenum disulfide and glass, calcium carbonate is difficult to dissolve in water, organic solvents and oils. If calcium carbonate is allowed to stand in the form of a mixture with water, organic solvents and oil, it tends to form a deposit. In order to keep the mixture in the best condition at the time of coating, it has to be stirred at all times.
Where steel products are subjected to cold working by various methods, it is usual to use a compound film such as of a phosphate, an oxalate and the like so as to enhance adherence and retention of a lubricant and also to prevent contact with tools, especially when the cold working is performed under such severe conditions that oil lubrication does not work effectively. The compound film is formed by pre-treatment and a soap lubricating film serving as a lubricant, both of which are used to lower the frictional force exerted between the tool and the material to be worked, thereby preventing the wear and seizure.
For instance, JP-B2 No. 4-4045 discloses a technique wherein a solution of a phosphate or an oxalate is fed onto the surface of a metal strip just prior to rolling to form a phosphate or oxalate film on the surface of the metal strip on the way of the cold working. The method proposed in this publication is effective in preventing seizure in cases where steel products are cold worked. However, it has little effect on preventing the seizure when steel products are hot rolled. This is because the phosphate or oxalate film is thermally decomposed when exposed to high temperatures ranging from 400.about.500.degree. C. or above, so that it is not possible to stabilize the phosphate or oxalate film on the surface of a steel product being rolled at a high temperature of approximately 800.degree. C. or above. Of course, scales are formed on the surface of the steel product which is being heated for the hot rolling, under which even if a phosphate or oxalate film is formed on the surface of the steel product, any anti-seizing effect cannot be expected. In the technique set out in the above-indicated publication, a solution of a phosphate or an oxalate is directly supplied to the surface of the steel product. If water which is the most inexpensive among the solvents is employed and a salt which is sparingly soluble in water (e.g. calcium oxalate and the like) is used, the sparingly soluble salt becomes clogged in pipes and the nozzle. Such an aqueous solution of the salt may not be supplied to the surface of the steel product even if the supply is desired. Thus, the technique disclosed in JP-B2 No. 4-4045 is applicable to cold working of steel products, but cannot be applied to hot working.